Among component elements of wireless communication equipment, power consumption is high in power amplifiers for transmitted signals used in the wireless communication equipment. Therefore, improvement in power efficiency of power amplifiers is a high priority need in the development of wireless communication equipment. In recent years, in wireless communication standards (for example, LTE (Long Term Evolution) standard), linear modulation is mainly used for spectral efficiency improvement. With linear modulation, demands regarding signal distortion are high. Therefore, in order to maintain linearity in a power amplifier, average output power is set to a value below saturated output power, so that instantaneous maximum output power (peak power output) is below saturated output. However, in a power amplifier, the more the average output power is lowered from saturated output power to a low ratio, the lower the ratio (power efficiency) of DC (direct current) power supply to the power amplifier with extracted transmission power. From the viewpoint of power saving, a decrease in power efficiency is not acceptable.
In order to solve this type of problem, various methods exist, where the transmission power of a power amplifier is digitally controlled. However, a problem occurs in that when transmission power is digitally controlled, quantization error is generated.
Patent Literature (PTL) 1 and Non-Patent Literature (NPL) 1 disclose a control system in which, by continuously (analogically) controlling the amplitude of RF signals received by a power amplifier carrying an LSB (Least Significant Bit), among a plurality of power amplifiers, quantization error generated by another power amplifier is corrected.
Patent Literature 2 discloses technology to continuously (analogically) control output power of a power amplifier so as to correct quantization error generated by another power amplifier, by continuously (analogically) controlling power supply voltage of a power amplifier carrying an LSB.
Non-Patent Literature 2 discloses a transmission apparatus that uses λ/4 lines and digitally controls transmission power of a power amplifier. Non-Patent Literature 3 discloses λ/4 lines that can be used as output combining elements.    PTL 1: Japanese Patent Kokai Publication No. JPH05-63458A    PTL 2: Japanese Patent Kokai Publication No. JPH04-321305A